Opening Remarks

• Noriyuki Sogabe (University of Osaka)

Room: Maskawa Hall

QCD critical point search: fluctuations and hydrodynamics

• Misha Stephanov (University of Illinois Chicago)

Room: Maskawa Hall

Simulating the critical dynamics of stochastic fluids

• Josh Ott (Massachusetts Institute of Technology)

Room: Maskawa Hall We describe numerical simulations of stochastic fluid dynamics near a critical point in the phase diagram. Using a Metropolis scheme, we evolve a conserved charge coupled to the momentum density of the fluid. This theory, known as model H, is expected to describe the nonequilibrium dynamics in the vicinity of a QCD critical point. We verify dynamic scaling near the critical point of a two and three-dimensional fluid and observe a crossover from the mean field value of the associated critical exponent to the expected model H value. This crossover is governed by the values of the correlation length and the renormalized sheer viscosity.

QCD Phase Diagram at Finite Volume via Lee–Yang Zeros and Lee–Yang Edge Singularities

• Tatsuya Wada (Kyoto University/YITP)

Room: Maskawa Hall We investigate the QCD phase diagram at finite volume using Lee–Yang zeros (LYZs) and Lee–Yang edge singularities (LYES). Properly accounting for finite-volume effects is a central issue in numerical lattice QCD analyses. In recent studies, the LYES has been employed to locate the QCD critical point; however, these approaches typically neglect finite-volume effects by assuming that LYZs obtained in finite-volume simulations can be directly identified with the LYES defined in the infinite-volume limit. In our recent work, we proposed a method based on LYZs that avoids this assumption. In this approach, we define the Lee–Yang zero ratio (LYZR) as the ratio of the imaginary parts of two distinct LYZs. According to finite-size scaling arguments, the LYZR evaluated at different volumes exhibits an intersection at the critical point, providing a robust criterion for its determination. In this study, we apply the LYZR method to a QCD effective model. We examine the qualitative relationship between LYZs and the LYES, and analyze the roles of finite-volume effects, mixing effects, and irrelevant operators. Finally, we discuss the feasibility of implementing the LYZR method in lattice QCD.

Bayesian Approach to RHIC Beam Energy Scan Program

• Chun Shen (Wayne State University)

Room: Maskawa Hall

Probing fractional quark charges and remnants of chiral criticality with fluctuations at the LHC

• Volodymyr Vovchenko (University of Houston)

Room: Maskawa Hall I will discuss how fluctuation measurements at the LHC can probe fractional quark charges at early stages of a heavy-ion collisions as well as possible remnants of chiral criticality. Utilizing a density correlations formalism we incorporate effects of local charge conservation, resonance decays, and hadronization of fractional charges to perform a Bayesian analysis of ALICE experimental data, revealing moderate evidence for the freeze-out of electric charge fluctuations in the QGP. I will then discuss an extension of this framework to higher-order net-proton cumulants, which are expected to be sensitive to remnants of chiral criticality. We calculate canonical baselines due to local baryon conservation and compare the resulting predictions with preliminary LHC data for O--O collisions

Search for QCD Critical Point at RHIC - A Status Report

• Nu Xu (CCNU/IMP)

Room: Maskawa Hall In this talk, I plan to discuss recent progresses in the study of the QCD phase structure with the measurements of collectivity and fluctuations in high-energy nuclear collisions at the Relativistic Heavy Ion Collider. Future studies, especially including the spin degrees of freedom at the newly constructed facilities, will be addressed.

Equation of State and Material Properties of Dense QCD Matter from (2+1)-Flavor Lattice QCD

• Jishnu Goswami (Bielefeld University)

Room: Maskawa Hall We present a lattice QCD determination of the equation of state of strongly interacting matter at finite baryon density using $(2+1)$-flavor simulations. The pressure, energy density, entropy density, and related bulk observables are obtained through a Taylor expansion in the baryon chemical potential around $\mu_B=0$. We further discuss material properties of QCD matter, including response coefficients relevant for the hydrodynamic description of heavy-ion collisions. These results provide first-principles input for characterizing the thermodynamics of dense QCD matter in the temperature range relevant to the QCD crossover.

Baseline fluid-dynamic modelling in the RHIC BES/FXT energy range

• Iurii Karpenko (FNSPE CTU in Prague)

Room: Maskawa Hall In this talk I discuss the status and recent progress in building and constraining multi-step fluid-based models (as well as hadronic transport) to simulate the non-critical baseline, or non-critical bulk medium dynamics of heavy-ion collisions at RHIC Beam Energy Scan energies, including the fixed-target regime.

In-in formalism and real-time dynamics in external electric field

• Shuhei Minato

Room: Maskawa Hall

Theoretical Challenges in the Search for the QCD Critical Point and the Chiral Phase Transition

• Noriyuki Sogabe (University of Osaka)

Room: Maskawa Hall

A new relativistic quntum molecular dynamics simulation for heavy-ion collisions

• Yasushi Nara (Akita International University)

Room: Maskawa Hall I present recent developments in a new relativistic quantum molecular dynamics (RQMD2) based on the time-dependent variational principle. It provides a simulation of heavy-ion collisions with a more accurate treatment of the equation of state.

Effective thermodynamic response of QCD matter at finite baryon density

• Li Yan (Fudan University)

Room: Maskawa Hall The correlations between mean transverse momentum and the total charged multiplicity in the relativistic high-energy ultra-central heavy-ion collisions have been recognized at the LHC energies. This type of correlation can be understood associated with the thermodynamic response between the variations of energy density and variations of pressure, in a uniform thermal system that effectively captures the identical thermodynamical properties, in particular, the speed of sound. In this talk, we extend the analysis to cases with finite baryon densities, regarding the QCD systems created at lower collision energies. We realize that the realistic fireball evolves approximately adiabatically, with a constant ratio of total entropy over total baryon number, $S/N_B$. Such a condition constrains the effective thermodynamic response, which not only modifies the speed of sound values, but also provide an essential mechanism that significantly suppresses the mean transverse momentum fluctuations in the vicinity of QCD critical end point.

Critical dynamics in an expanding system with baryon diffusion and chiral condensate

• Azumi Sakai (Nagasaki Institute of Applied Science)

Room: Maskawa Hall We construct a framework to describe the dynamics of critical fluctuations including the relaxation time for baryon diffusion current and the coupling of the chiral condensate $\sigma$ to the baryon density $n_B$. We apply this to one-dimensionally expanding system with the QCD critical point and investigate the effect of the relaxation time and the mode coupling. We analyze the spacetime evolution and correlation functions of the fluctuations and suggest the importance of dynamical effects of the chiral condensate and baryon diffusion current in identifying critical-point signals in heavy-ion collisions. This talk is based on the paper: Phys. Rev. C 112, 024907 (2025)

Dense QC2D2 with uniform matrix product states

• Kohei Fujikura (YITP)

Room: Maskawa Hall We explore $(1+1)$-dimensional cold and dense single-flavor $\mathrm{SU}(2)$ gauge theory using uniform matrix product states. Ground states are obtained by minimizing the Hamiltonian using variational uniform matrix product states, both with and without a baryon chemical potential. Several thermodynamic quantities, including the pressure and the expectation value of the baryon density, are computed with remarkable precision. We further compute the quark distribution function in momentum space. We find that the infrared behavior at finite baryon density is described by a Tomonaga--Luttinger liquid. The Luttinger parameter is determined under certain assumptions.

Bulk observables constraining the QCD equation of state and phase structure

• Takafumi Niida (University of Tsukuba)

Room: Maskawa Hall

Boost invariant solution of dissipative spin hydrodynamic framework and thermal dilepton production

• Sejal Singh (Department of Physics, Birla Institute of Technology and Science Pilani, Pilani Campus, Pilani, Rajasthan-333031, India)

Room: Maskawa Hall Heavy-ion collisions have been an excellent tool for studying the nature of the strong force. In non-central heavy-ion collisions, a large amount of angular momentum is generated during the collision process. This angular momentum can induce a non-zero vorticity in the medium, leading to the spin polarization of the produced hadrons. Experimental evidence for such vortical structures has been observed in relativistic heavy-ion collisions [1]. Various theoretical approaches, including spin hydrodynamics, have been developed to describe this polarization phenomenon [2-4]. The hydrodynamic evolution of such a relativistic spin-fluid should ensure conservation of total angular momentum tensor ($\partial_{\lambda}J^{\lambda\alpha\beta}=0$), along with the conservation of energy-momentum tensor ($\partial_{\mu}T^{\mu\nu}=0$) and baryon current ($\partial_{\mu}J^{\mu}=0$). Spin hydrodynamics promotes total angular momentum to a hydrodynamic variable with its own conservation laws and gradient expansions. \begin{align} & J^{\lambda\alpha\beta}=\mathcal{O}(1)+\mathcal{O}(\partial)+\mathcal{O}(\partial^2)+... \end{align} We investigate the solution of spin hydrodynamic equations for a boost invariant system by taking the spin chemical potential as a leading-order ($\mathcal{O}(1)$) hydrodynamic variable. For a symmetric energy–momentum tensor and an independently conserved spin tensor, we derive the coupled evolution equations governing the temperature evolution of the medium and the independent components of the spin chemical potential, including dissipation from both viscous and spin diffusive currents. We also consider a spin dependent equation-of-state to solve the spin hydrodynamic equations. We solve the spin hydrodynamic equations, including dissipative effects, for a longitudinally expanding boost-invariant system. We find that for Bjorken flow only the magnetic-like components of the spin chemical potential affect the proper time evolution of the system. We find that the spin transport coefficients crucially determine the proper time evolution of the spin chemical potential, i.e., the evolution of the spin tensor. Due to dissipative effects, the longitudinal component of spin chemical potential survives for a longer duration, whereas transverse components decay more rapidly. We also find that a non-vanishing spin chemical potential and spin diffusion affect the system's temperature evolution. Using the temperature evolution in the dissipative spin-hydrodynamic framework, we estimate the thermal dilepton production rate from quark-antiquark annihilation. Thermal dileptons are considered an excellent probe of medium temperature, and capture the crucial information of the system evolution. We find that the inclusion of spin dynamics enhances the dilepton production yield, and the magnitude of the enhancement depends on the spin transport coefficients. Various research groups are working on the development of numerical tools for dissipative spin hydrodynamics, and these results can serve as a test for such numerical simulations. References [1] STAR Collaboration, B. I. Abelev et al., “Global polarization measurement in Au+Au collisions,” Phys. Rev. C 76 (2007) 024915, arXiv:0705.1691 [nucl-ex]. [Erratum: Phys.Rev.C 95, 039906 (2017)]. [2] S. Bhadury, W. Florkowski, A. Jaiswal, A. Kumar, and R. Ryblewski, “Dissipative Spin Dynamics in Relativistic Matter,” Phys. Rev. D 103 no.1, (2021) 014030, arXiv:2008.10976 [nucl-th]. [3] R. Takahashi, M. Matsuo, M. Ono, K. Harii, H. Chudo, S. Okayasu, J. Ieda, S. Takahashi, S. Maekawa, and E. Saitoh, “Spin hydrodynamic generation,” Nature Physics 12 no. 1, (2016) 52–56. [4] W. Florkowski, B. Friman, A. Jaiswal, R. Ryblewski, and E. Speranza, “Relativistic hydrodynamics with spin,” Nucl. Phys. A982 (2019) 523–526, arXiv:1807.04946 [nucl-th].

Nature of the soft mode of QCD critical point and effects on dilepton production rate and electric conductivity

• Teiji Kunihiro (YITP, Kyoto U.)

Room: Maskawa Hall The QCD critical point (QCD-CP) of a second-order nature comes to exist when an explicit chiral symmetry breaking is present at finite baryon density where the charge-conjugation symmetry is lost. Thus, the order parameter of it is a linear combination of the baryon density and the chiral scalar condensate, but not the pure scalar chiral condensate. Accordingly, the dynamical fluctuations of the order parameter in the normal phase has a dominant strength in the space-like region coming from the collective particle-hole excitation corresponding to density fluctuations; the peak energy of the collective mode goes down (softens) as the system approaches the QCD-CP in the normal phase and eventually vanishes there. This is the soft mode of the QCD-CP. In this talk, employing Nambu-Jona-Lasinio model, we show that the photon self-energy in the medium is modified by the soft mode so significantly that the dilepton-pair production rate as well as the electric conductivity are anomalously enhanced near the critical point in the normal phase, through an Aslamazov-Larkin-like and other related processes, known to cause the 'para conductivity' in metal superconductors above the critical temperature.

Hydrodynamic Consequences of the QCD Critical Point

• Chiho Nonaka (Hiroshima University)

Room: Maskawa Hall Criticality in QCD manifests itself in two complementary ways: through enhanced fluctuations and through modifications of the macroscopic evolution of the medium. While fluctuation observables have long been regarded as key signatures of the QCD critical point, the hydrodynamic response of QCD matter to criticality may provide equally important information. I will discuss recent progress in understanding hydrodynamic evolution near the QCD critical point, with particular emphasis on the focusing of hydrodynamic trajectories and its implications for heavy-ion collision phenomenology.

Is there any critical behavior in heavy quark dynamics?

• Yukinao Akamatsu (National Institute of Technology, Matsue College)

Room: Maskawa Hall We study how heavy quark dynamics are affected by critical fluctuations near the QCD critical point at finite temperature and density. The momentum diffusion constant scales as $\kappa \propto \xi^{z-3-\eta}$, and we also examine the enhancement of the heavy quark spectral width near criticality. In Model H, the critical enhancement of $\kappa$ is negligible, while in Model B ($z \simeq 4$) it becomes singular, $\kappa \propto \xi$. These results demonstrate the sensitivity of heavy quark transport to the dynamical universality class near the QCD critical point.

Relativistic Hydrodynamics with a Critical Point

• Isabella Danhoni (UIUC)

Room: Maskawa Hall

cumulant expansion of thermodynamic potential in heavy-quark QCD

• Takahiro Doi (Kyoto University)

Room: Maskawa Hall We investigated the thermodynamic potential of heavy-quark QCD. We perform the heavy-quark expansion (hopping parameter expansion) for the QCD action and the cumulant expansion for the thermodynamic potential. We examined the convergence of the cumulant expansion for the thermodynamic potential using two independent methods. First, we estimated the convergence radius directly from the coefficients of the cumulant expansion. Second, we determined the radius of convergence using the method of Lee–Yang zeros. The two results are consistent with each other and indicate that heavy-quark QCD exhibits three distinct regimes: a pure quark regime, a pure baryon regime, and a mixed regime.

Fluctuation Signatures from the Equation of State to Proton Factorial Cumulants

• Jamie Karthein (Texas A&M University)

Room: Maskawa Hall As an important set of thermodynamic quantities, knowledge of the equation of state over a broad range of temperatures and chemical potentials in the QCD phase diagram is crucial for our understanding of strongly-interacting matter. With the equation of state, important questions about QCD phase structure can begin to be addressed, such as whether there is a critical point in the QCD phase diagram. In addition, to draw meaningful conclusions from experimental data, a theoretical framework is needed to link QCD thermodynamics with the particle spectra and correlations observed in the detectors. In this talk, equations of state from first-principles and effective theories will be discussed in order to understand how QCD thermodynamics is affected by the presence of a critical point. Furthermore, the maximum entropy approach is used to freeze-out the fluctuations in order to make estimates for factorial cumulants of proton multiplicities, assuming thermal equilibrium, for a family of EoS with a 3D Ising-like critical point, varying the microscopic inputs that determine the strength and structure of the critical features. We quantify the effect of the non-universal mapping parameters, and the distance between the critical point and the freeze-out curve, on the factorial cumulants of proton multiplicities measured in experiment.

Extracting QCD baryon susceptibilities from proton fluctuations at BES energies

• Gregoire Pihan (University of Houston)
• Roman Poberezhniuk
• Volodymyr Vovchenko (University of Houston)

Room: Maskawa Hall We present a new framework for connecting proton fluctuation measurements in relativistic heavy-ion collisions to QCD baryon-number susceptibilities. The pipeline combines viscous hydrodynamic simulations with a maximum-entropy construction of proton and antiproton fluctuations from baryon-number fluctuations on the particlization hypersurface. Finite experimental acceptance and exact global charge conservation are incorporated using the newly developed Subensemble Acceptance Method, SAM 3.0. Applying this framework to proton cumulants and factorial cumulants in Au+Au collisions from the RHIC Beam Energy Scan, we perform a Bayesian extraction of the second-order baryon-number susceptibility. We find a growing tension with lattice-QCD-based 4D Taylor-expansion predictions at larger baryon chemical potential, suggesting that proton fluctuation data may provide direct sensitivity to changes in the underlying QCD thermodynamics.

Discussion 1 Room: Maskawa Hall

Discussion 2 Room: Maskawa Hall